Fix detection of the result type of strerror_r().

The method we've traditionally used, of redeclaring strerror_r() to
see if the compiler complains of inconsistent declarations, turns out
not to work reliably because some compilers only report a warning,
not an error.  Amazingly, this has gone undetected for years, even
though it certainly breaks our detection of whether strerror_r
succeeded.

Let's instead test whether the compiler will take the result of
strerror_r() as a switch() argument.  It's possible this won't
work universally either, but it's the best idea I could come up with
on the spur of the moment.

Back-patch of commit 751f532b.  Buildfarm results indicate that only
icc-on-Linux actually has an issue here; perhaps the lack of field
reports indicates that people don't build PG for production that way.

Discussion: https://postgr.es/m/10877.1537993279@sss.pgh.pa.us